1. Field of the Art
The present invention relates to a process for producing methylamines by a gas phase catalytic reaction of methyl alcohol with ammonia. More particularly, the present invention relates to a process for producing methylamines characterized in that the catalytic reaction is carried out in two steps by using, different catalysts.
Methylamines are usually prepared by reacting methanol with ammonia in gas phase in the presence of a solid acid catalyst (hereinafter referred to as conventional catalyst) such as alumina and silica at an elevated temperature. In this catalytic reaction, part or all of the hydrogen atoms in the ammonia are replaced with a methyl group, whereby three types of methylamines, i.e., monomethylamine (hereinafter referred to as MMA), dimethylamine (hereinafter referred to DMA) and trimethylamine (hereinafter referred to as TMA) are simultaneously produced. These methylamines are separated from the reaction mixture. DMA which is in greatest commercial demand among these methylamines is utilized as a final product, while MMA and TMA which are in less commercial demand are mostly transferred to the reaction system for reuse. In this case, the methylamine mixture (MMA+TMA) transferred to the reaction system contains the non-reacted ammonia because an excessive amount of ammonia is usually used in order to enhance the equilibrium conversion of methanol and the production ratio of DMA during the catalytic reaction. Therefore, it may be stated as a conclusion that methylamines are prepared by reacting together methanol, ammonia and a methylamine mixture containing TMA.
DMA is separated from the reaction product by distillation. However, it is not easy to distill DMA. Since TMA contained in the reaction product forms azeotropic mixtures with ammonia, other amines and the non-reacted methanol, respectively, recovery of DMA or a DMA-TMA azeotropic mixture, recovery of DMA from the DMA-TMA azeotropic mixture, and recovery of MMA and TMA become necessary, which unavoidably causes the operation to be complicated, the apparatus to be large-sized, and the consumption of energy to be increased.
Accordingly, if the formation of DMA is promoted in the synthetic reaction system while the formation of TMA is suppressed, reduction in the utility cost of the DMA refining process and decrease in the size of the apparatus can be directly attained.
However, the final ratios of formation for three types of methylamines are thermodynamically determined. That is, the higher the temperature and the higher the ratio of the number of nitrogen atoms to the number of carbon atoms, N/C, in the reaction mixture, the higher is the ratio of formation of DMA and the lower is the ratio of formation of TMA. For example, when the reaction temperature is 400.degree. C. and the N/C is 2.0, the ratio of formation at equilibrium of each methylamine is thermodynamically calculated as follows: MMA=0.288, DMA=0.279 and TMA=0.433. Since the rate of formation of TMA is relatively high in the presence of the conventional catalyst, the ratio of formation of DMA or the ratio of formation of DMA/TMA never exceeds the above mentioned equilibrium value throughout the reaction process. Therefore, a large amount of TMA and MMA after they are separated from the reaction product should be recycled to the reaction system as described hereinabove. The ratio of formation of DMA can be increased by increasing the reaction temperature or the N/C ratio so as to shift the reaction equilibrium itself. In this case, however, the increase in the reaction temperature results in an increase in the formation of impurities and the increase in the N/C ratio causes the non-reacted ammonia to be recycled to be increased, which requires a large-sized apparatus. For these reasons, these approaches are not always advantageous from an economic point of view.